Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a source of information-carrying optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the optical signals and demodulating the information they carry. The signals are typically within a wavelength range favorable for propagation within silica fibers, and preferably comprise a plurality of wavelength distinct channels within that range.
The optical fibers are thin strands of glass of composition capable of transmitting optical signals over long distances with very low loss. They are small diameter waveguides characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the fiber with low attenuation. Typical fibers are made of high purity silica with Germania doping in the core to raise the index of refraction. A transmission line may include many long segments of such fiber separated by intermediate nodes for adding or dropping off signal channels.
Despite significant progress in reducing the attenuation characteristics of optical fibers, signals transmitted through them are attenuated by the cumulative and combined effect of absorption and scattering. Consequently long distance transmission requires periodic amplification.
One approach to optical amplification utilizes Raman effect amplification. In the Raman effect, light traveling within a medium is amplified by the presence of lower wavelength pump light traveling within the same medium. The gain spectrum of a silica fiber pumped by a monochromatic pump was first measured in 1972. The maximum gain occurs when the signal is at a frequency approximately 13 THz lower than the frequency of the pump. The frequency (or wavelength) difference between the pump and the frequency (or wavelength) of maximum gain is often referred to as the Stokes shift, and the amplified signal is referred to as the Stokes wave. Use of a pump that is detuned from the signals by about one Stokes shift (1/2 the Stoke shift to 3/2 the shift) is referred to as first-order Stokes pumping.
It has also been observed that the gain is significantly larger for a co-polarized signal and pump. This polarization sensitivity can be eliminated if the pump is depolarized, polarization-scrambled on a sufficiently fast time scale or composed of two equally powerful polarized pumps that are polarization multiplexed. See, for example, U.S. Pat. No. 4,805,977, issued to Y. Tamura et al and entitled "Optical Coupler for Optical Direct Amplifier".
Signal amplification utilizing distributed first order Raman effect amplifiers is described in U.S. Pat. No. 4,616,898 issued to John W. Hicks, Jr. on Oct. 14, 1986. The Hicks et al. system disposes a plurality of optical Raman pumps at spaced intervals along the transmission line. These pumps inject into the optical fiber optical pump light at a wavelength shorter than the signal wavelengths by a Stokes shift, so that the presence of the pump light amplifies the lower wavelength signals by the first order Raman effect.
The use of first-order Stokes pumping has several limitations. The power of a strong Raman pump in amplifying a weak signal will always decrease exponentially with of distance as the light propagates into the transmission fiber. This means that regardless of how powerful the pump, most of the amplification occurs relatively near the point where the pump is injected into the fiber (typically within 20 km). This significantly limits the improvement in the signal-to-noise ratio that the Raman pump can induce. As the pump power is increased, Rayleigh scattering of the signal limits the improvement in the signal-to-noise ratio.
In some systems, the distributed Raman amplifiers may be followed by erbium amplifiers. The increased signal power at the input of the erbium amplifier causes the erbium amplifier to have a higher noise figure than it would in the absence of distributed Raman amplification. This effect increases the noise figure of the composite erbium/Raman amplifier and therefore decreases the improvement in the signal-to-noise ratio.